WEDNESDAY, July 2, 2014 (HealthDay News) -- Use of fertility drugs doesn't appear to increase a woman's long-term risk of breast, ovarian and uterine cancers, new research indicates.

The findings are "generally reassuring," said study co-author Dr. Humberto Scoccia, of the University of Illinois at Chicago. Still, he urged that women who use fertility drugs be closely monitored as they age.

Nearly 10,000 women who underwent infertility treatment in the United States between 1965 and 1988 were followed for 30 years for the study. During the follow-up, 749 cases of breast cancer, 119 cases of uterine cancer and 85 cases of ovarian cancer were diagnosed among the women.

Fertility drugs increase levels of the hormones estradiol and progesterone, which have been associated with these women's cancers. Clomiphene citrate was the most commonly used fertility drug until the 1980s, the study authors noted.

The researchers found "little evidence" that the use of conventional fertility drugs increased the long-term risk of breast, uterine or ovarian cancers.

However, extended use of clomiphene citrate was associated with a higher risk of breast cancer among women who used the fertility drug for 12 cycles or more, Scoccia's team said.

In general, the use of gonadotrophins -- follicle-stimulating hormones -- was not associated with increased cancer risk, except in women who remained childless after undergoing fertility treatment, the investigators found.

The findings, presented this week at the annual meeting of the European Society of Human Reproduction and Embryology in Munich, should be considered preliminary until published in a peer-reviewed medical journal.

Although the findings don't support "a strong relationship" between the use of fertility drugs and breast, uterine and ovarian cancers, the researchers recommend long-term monitoring for several reasons. These include the "relatively young age" of the study population and the later peak occurrence of most of these cancers.

Also, they said the number of patients using gonadotrophins for ovarian stimulation, particularly with in vitro fertilization, increased substantially after the mid-1980s.